Conventionally, keyhole-type welding is widely known as a method for securing relatively large depth (for example, not less than 0.3 mm) of weld penetration in laser beam welding.
The keyhole-type welding is a welding method using a laser beam having high power density, and realizes relative large depth of weld penetration by utilizing a hole (keyhole) formed in a molten pool by pressure of metal vapor.
The keyhole-type welding is used for products whose welded portion requires long-term durability, such as a case of a car battery.
However, the keyhole-type welding may cause a defect (underfill) that the welded portion is recessed when molten metal is blown away because of high output value of the laser beam.
On the other hand, if the output value of the laser beam is reduced in order to minimize occurrence of the underfill, the keyhole may not be formed, and thereby the desired depth of weld penetration cannot be secured (penetration shortage occurs).
When the above-mentioned defect occurs in the welded portion of the product, desired durability cannot be secured. Therefore, the welded portion of the product must be inspected.
Generally, the inspection of the welded portion of the product is performed by an operator with the naked eye. Therefore, the inspection requires a great deal of time.
The Patent Literature 1 discloses a technique for shortening the time required for the above-mentioned inspection.
The technique disclosed in the Patent Literature 1 detected a defect of a welded portion on the basis of intensity of plasma light radiating from a base metal during laser beam welding, and intensity of reflected light of a laser beam.
However, it is difficult to detect the above-mentioned defects such as the underfill using the technique disclosed in the Patent Literature 1 because of the following reasons:
The plasma light is generated by heating components of the base metal and shielding gas, and has no correlation with the underfill and the keyhole.
The reflected light is widely varied by a state of a reflecting surface, behavior of a molten pool and the like, and has low correlation with the underfill and the keyhole.